Almost overnight, Mediapart has put internet journalism on the map in France, where web-only news sites were widely viewed, not least among journalists, as marginal adventures trailing in the wake of traditional media.

Launched in 2008 by four senior staffers from Le Monde, led by the paper's former editorial director Edwy Plenel, it offers subscribers a daily menu of left-leaning, alternative news reporting with an accent on campaigning and investigative journalism. The standard monthly subscription rate is €9 (£7.50).

Mediapart has 25 editorial staff (aged between 25 and 60), who earn salaries equal to the print jobs they quit immediately before joining.

Initial investment was just short of €3m (about £2.5m): €1.3m from the founders, the rest made up of two major shareholders and a consortium of minority investors who include Xavier Niel, one of the three-part consortium that last month bought Le Monde.

But until June, the site was struggling with a total of just 25,000 subscribers, half the amount it needs to break even, which it has forecast it will do in 2012. Butlergate – the secret audio recordings of apparent skulduggery in the drawing room of one of France's richest households – may now change all that.

"The tapes drew in about 5,000 new subscribers in June, many of whom would otherwise shy away from crossing the payment threshold," Sophie Dufau, Mediapart's assistant editor, told mediaguardian.co.uk. June page views rose by 1.5m on May, to reach almost 4.1m.

While the leading French weekly magazine Le Point separately also got hold of the tapes, Mediapart demonstrated the edge internet journalism has over print - immediacy and sound.

The conversations are between the L'Oréal heiress Liliane Bettencourt and her entourage of lawyers and financial managers, covertly taped over a 12-month period by her major-domo. In the background is clinking chinaware and occasional snoring, apparently from Bettencourt.

"We spent several days with the tapes before posting the audio," said Dufau. "We decided to use only those extracts that were clearly of public interest."

That was also the view of French judges who threw out an invasion of privacy suit launched by Bettencourt's legal team.

From 16 June, the website ran exclusive extracts in instalments before Le Point finally caught up by publishing the transcripts in full in its 1 July edition, when over-the-counter sales rose by 25% to 100,000.

Mediapart's investigations culminated last week in its biggest coup yet; an interview with Bettencourt's former book-keeper, who alleged the L'Oréal owner and her now-deceased husband illegally financed Sarkozy's 2007 presidential campaign. That launched a major police investigation, and became news headlines around the world.

As in last year's Westminster expenses scandal, Butlergate has proved that quality investigative journalism pays.

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